A slip clutch is well-known as a mechanism capable of transmitting a torque from an input shaft to an output shaft while limiting transmission of an excessive input torque when such an excessive input torque works (see patent documents noted below for example). The slip clutch is configured to transmit a torque through friction between a friction plate spline-coupled to the input shaft and a friction plate spline-coupled to the output shaft and limit transmission of the torque by causing the friction plates to slip relative to each other when an excessive input torque works.
Presently, however, a problem exists that it is difficult to reliably limit an output torque to a desired magnitude of not more than a prescribed value because of instability in friction coefficient and depending on some operating conditions even when braking materials having preferable properties are combined with each other. Particularly, the slip clutch undeniably allows a surge or overshoot phenomenon such that the output torque exceeds the prescribed value undesirably to occur before and after occurrence of a slip.
In addition, there exists a request to make a frictional torque produced by the friction plates upon occurrence of the slip as small as possible. This is because generation of heat and the amount of wear increase as the frictional torque produced upon occurrence of the slip increases.
An intended object of the present invention having been made in view of the foregoing circumstances is to realize a high-precision torque limiting mechanism capable of limiting the output torque to a magnitude of not more than the prescribed value, as well as to reduce the frictional torque produced upon occurrence of the slip.    Patent Document 1: Japanese Patent Application No. 02005-327332    Patent Document 2: Japanese Patent Laid-Open Publication No. 2002-031156